Add merge_index bug fix to release notes
Add merge_index bug fix to release notes

amyvmiwei/riak,biddyweb/riak,elkingtonmcb/riak,GabrielNicolasAvellaneda/riak,xuejianhai/riak,laurenrother/riak,yangchengjian/riak,glickbot/riak,basho/riak,authbox-lib/riak,DavidAlphaFox/riak